The present invention relates to an apparatus for measuring blood flow, and more particularly to an apparatus for measuring blood flow for noninvasively obtaining blood flow information.
Apparatuses employing laser beam irradiation for noninvasively measuring data relating to blood flow within in vivo tissue are well known. Most of these prior art apparatuses project a laser beam onto the in vivo tissue so as to obtain a speckle pattern by interference among the rays of light scattered by corpuscles within the tissue. The speckle pattern varies with the movement of the corpuscles and it is possible to obtain a speckle signal proportional to the corpuscle velocity by, for example, measuring the variation in intensity of the light reflected from a single point. Thus in apparatuses of this type, data relating to blood flow is obtained from variations in the speckle signal.
In one such prior art method of obtaining blood flow data, the measurement is carried out by using optical fiber probes for detecting time-course variation in the intensity of light reflected at selected measurement points. This method is directed solely to obtaining data in respect of a single point and, therefore, does not allow observation of blood flow distribution over a given area.
There is also known a method of measuring blood flow distribution by two-dimensionally scanning the in vivo tissue with a laser beam. This method is described in Japanese, Patent; Public Disclosure Sho 63(1988)-214238 which discloses using a mirror to scan the in vivo tissue surface with a laser beam spot that has been linearly expanded by a cylindrical lens. The reflected scattered light is detected with an image sensor. Also, this method is described in Japanese Patent Public Disclosure Sho 64(1989)-37931 which discloses using a light deflecting element for reciprocally scanning the in vivo tissue surface with a light spot at a prescribed, scanning period. Blood flow data is derived from the difference between two light signals obtained from the same spot at two points of time separated by a prescribed time interval.
Since these prior art measurement methods require continuous scanning involving the reciprocal movement of irradiating light with respect to the in vivo tissue surface, a prerequisite of their use is that the in vivo tissue under examination be maintained stationary throughout the measurement. Specifically, since the data is compared and calculated at every measurement point within the measurement region during every scanning cycle, any shift in the measurement points would affect all of the measurement data and immediately deprive it of reliability.
Moreover, while the two-dimensional scanning enables spatially continuous measurement, it is intermittent in terms of time from the viewpoint of any given single scanned point. As a result, the frequency of the speckle signal obtained is dependent on the scanning frequency. Where measurement is conducted with respect to rapid blood flow such as encountered in the eye fundus, for example, the speckle signal falls in a high frequency band and, with the conventional methods, it becomes necessary to increase the scanning speed accordingly. This makes it necessary to use a more complex apparatus.